This invention relates to an improvement in the structure of bipolar electrode-type, filter press solid polymer electrolyte electrolysis cells. More particularly it relates to those of such cells which employ permselective ion exchange membranes having an electrocatalytic material bonded to or embedded in the membrane and acting as anodes or cathodes. Such cells are particularly useful in the electrolysis of aqueous solutions of alkali metal chlorides; especially in the electrolysis of aqueous solutions of sodium chloride (sodium chloride brine). The cell structure may also be used in electrolyzing other solutions to make products such as potassium hydroxide, iodine, bromine, bromic acid, persulfuric acid, chloric acid, adiponitrile and other organic compounds made by electrolysis.
The unitary filter press central cell element used in the present invention decreases the cost of manufacture of the cell units, decreases the labor required to assemble them, simplifies their manufacture, greatly reduces the warpage of the cell unit parts, and provides a much sturdier cell structure than do bipolar, filter press cells of the prior art.
Reducing the warpage of cell structure allows the cell to be operated more efficiently; i.e., produce more units of electrolysis products per unit of electricity. Reducing the warpage reduces the deviation from design of the gap width between the current collectors and/or anode and cathode of each electrolysis cell. Ideally this gap width is uniformly the same between the current collectors and/or anode and cathode in order to have a uniform current density spread between the faces of the cell electrodes. Among other things, structural warpage causes deviation of this gap resulting in some parts of the current collector and/or anode and cathode being closer together than others. At these locations, the electrical resistance is less, the electrical current is more, and thus the electrical heating is greater. This electrical heating is sufficient in many instances to cause damage to the membrane at these locations. These locations of unacceptably high electrical current concentration and high heat are referred to herein as "hot spots".
To avoid these hot spots, the prior art has had to design its cell structures with a greater than desired gap width between the current collectors and/or anode and cathode of each electrolysis cell. This, of course, increases the cell operating voltage and decreases the cell operating efficiency. Complexity of design and fabrication is another drawback of those cells.
Except for the structures used fdr the terminal cells of a bipolar filter press cell series, the structures for the intermediate cells in the series are like, repetitious, cell structural units which are squeezed together. Examples of such cells operated in a cell series are disclosed in Seko, U.S. Pat. Nos. 4,111,779 (Sept. 5, 1978) and in Pohto, 4,017,375 (Apr. 12, 1977). These patents are herein incorporated by reference for purposes of showing representative prior art and for showing how bipolar filter press cells are formed into and operated in a cell series.
At this point, a clarification should be made about confusing nomenclature sometimes encountered when speaking of a series of bipolar filter press cells. The problem involves the nomenclature often encountered when dealing with the repeating electrolysis cells and the repeating cell structure units used to house these repeating electrolysis cells. In the electrolysis cells there is a membrane planarly disposed in or about the center of each electrolysis cell between a parallel anode and cathode. The membrane divides the electrolysis cell itself into an anode and cathode compartment. However, in appearance in a cell series the membrane often appears to be the division line between repeating cell units. In fact, it often is located at the division between repeating cell structures in the series, but not at the line separating different electrolysis cells. This comes about because the repeating cell structures are situated between and around parts of adjacent, but different, electrolysis cells. Such a repeating cell structure includes structure which defines the periphery of the cathode compartment of one of two adjacent electrolysis cells. This repeating cell structure includes structure which defines the periphery of the anode compartment of the other of the two adjacent electrolysis cells and the barrier structure separating the two electrolysis cells. So the anode compartment and the cathode compartment associated with a given repeating structural unit are compartments of adjacent, but different, electrolysis cells.
These repeating cell structures include several other structural elements which will be discussed below. Herein this repeating structural unit will be referred to as a "bipolar electrode-type, filter press-type solid polymer electrolyte electrolytic cell unit". As used with the present invention, this cell unit is referenced in the drawing by reference number 10.
There are other structural elements included in a bipolar electrode-type, filter press-type electrolytic cell unit besides the electrolyte compartments peripheral structure and the electrolyte impervious central barrier. These include an anode, a cathode, an anode stand-off means, a cathode stand-off means, and an electrical current transfer means.
The central barrier separates the anode compartment of one adjacent electrolysis cell from the cathode compartment of the other adjacent electrolysis cell.
The anode and cathode are spaced from and spaced on opposite sides of the central barrier by the anode and cathode stand-off means, respectively. This spacing is provided so as to provide room for the electrolyte and electrolysis products to circulate in the space between the electrodes and their central barriers.
The anode stand-off means and cathode stand-off means most often also serve as the electrical current means used to electrically connect the anode on one side of the barrier with the cathode on the opposite side of the barrier. This connection is made through the barrier.
The anode and cathode are usually of the "flat plate" type. That is, they present a planarly disposed working surface, or assembly of surfaces, to their respective membranes. They are most often parallelly disposed to their respective membranes, to the axis plane of the central barrier, and to each other. Also the anode and cathode are usually made of a foraminous metal.
The anode compartment is defined by the space between the central barrier and the membrane disposed on the anode side of the central barrier as well as the structure fitted around and between the periphery of this membrane and central barrier. Note, the anode is disposed within the anode compartment by definition. Likewise the cathode compartment is defined as the space between the central barrier and the membrane on the cathode side of the central barrier and by the peripheral structure fitted around and between the periphery of the central barrier and the membrane on the cathode side of the central barrier. Again the cathode is in the cathode compartment by definition.
The anode and cathode of a repeating unit structure (along with the central barrier and the electrical connecting means which electrically connects the anode to the cathode through the central barrier) are, of course, often referred to as a "bipolar electrode". This is because, in effect, this connection of structure series is as an anode in one electrolysis cell and a cathode in another electrolysis cell.
The above features of a flat plate bipolar electrode-type, filter press-type electrolytic cell unit can also be observed in the following references: U.S. Pat. Nos. 4,364,815; 4,111,779; 4,115,236; 4,017,375; 3,960,698; 3,859,197; 3,752,757; 4,194,670; 3,788,966; 3,884,781; 4,137,144; and 3,960,699.
A review of these patents discloses the above described structural elements in various forms, shapes and connecting means.
What is surprising to one not skilled in this art is the complexity of connections of these parts as well as the large number of parts required for what seems to be a relatively simple structural assembly problem. Of course, to those skilled in the art this complexity is well understood as the outgrowth of trying to make profitable commercial cell structures for use with the relatively new permselective ion-exchange membranes and the extremes of corrosive conditions extant between the anode and cathode compartments. These membranes operate best at elevated temperatures and high caustic concentrations, e.g., above about 80.degree. C. and about 22% caustic catholyte concentrations. This compounds the problems of constructing profitable cells.
The problem centers around finding an affordable anode material and other materials which can withstand the extremely corrosive conditions of the anode compartment. For profitable, commercial operations, titanium is the material which has been found which has the most promise for profitable use.
However, there is a great disadvantage in the use of titanium with other metals suitable for use in the anode compartment. This is titanium's inability to form a good weld with ferrous materials and most other materials. This is most unfortunate because steel has been used quite successfully for many years as the cathode material.
The major reason for the complexity existing in the connections as well as the reason for having so many connections and so many separate parts in each filter press cell unit of the prior art stems from the necessity of using titanium coupled with the relatively high cost of titanium with respect to the cost of steel coupled with the necessity of establishing a very low electrical resistance connection between the anode and the cathode. The present invention greatly reduces the number of connections, number of separate parts, and the problems they cause. Further discussion of these problems will be better appreciated by perusing the prior art.
As stated above, one of the main problems is that titanium cannot be successfully welded directly to steel. See Seko, U.S. Pat. No. 4,111,779 at Column 1. Also see Mitchell, D. R.; Kessler, H. D.; "The Welding of Titanium to Steel", Welding Journal (December 1961). In the Seko patent, titanium is joined to steel by explosion bonding steel plate to titanium plate. In the Mitchell et al Welding Journal article, titanium is indirectly welded to steel by welding through a vanadium intermediate placed between the steel and titanium.
The prior art discloses complex and elaborate schemes devised to electrically and/or mechanically connect the different parts of the cell wherein titanium and titanium alloys are employed. Particularly is this complexity seen to be true with respect to the parts herein referred to as stand-offs which connect the "flat plate" anode and cathode of a bipolar electrode structure to an electrically conductive central barrier at a spaced distance from the central barrier; e.g. Seko U.S. Pat. Nos. 4,111,779 and Ichisaka et al, 4,194,670. Other stand-offs are used to support the flat plate electrodes and to electrically and mechanically connect them through holes in a non-conductive central barrier, e.g., Stephenson III, et al, U.S. Pat. Nos. 3,752,757 and Bortak, 3,960,698. It will be noticed that in these connections, welds and/or bolts are used to connect the stand-offs to the electrodes and then again to the central barrier or to opposing stand-offs passing through the central barrier. Many problems are associated with these many connections. These problems would not be so formidable if only a few connections were required for each of the many cells in a series, but many are required for each cell to get adequate electrical current distribution.
The present invention reduces these problems by eliminating many of these connections. It does this by integrally casting these stand-offs with the central barrier. Moreover, the connections used to connect the central barrier to the peripheral structure of the anolyte and cathode compartment are also eliminated by integrally casting these structures with the central barrier.
Other problems associated with having so many such conuections include unequal electrical current transfer, warpage of parts, and creation of more stress points in the titanium. Such stress points are subject to attack by atomic hydrogen as well as increased susceptibility to normal chemical corrosion and galvanic corrosion.
The electrical transfer capabiity of a bolted connection is dependent upon the sufficiency of the friction contact between the threads of the two mating threaded pieces. Many bolts are used in making the connections for each bipolar unit when they are depended upon to connect the electrodes and/or stand-offs. They are depended upon to carry equal amounts of current to avoid "hot spots" on the electrodes and adjacent membranes. However, this would require perfect equality of mating of all threaded surfaces. Perfection can not be closely approximated in these cells without going to extraordinary costs. Hence, "hot spots" do occur, and if they do not burn the membrane, they at least cause distorted electrolysis reaction rates across the face of the electrode.
As to welded connections, electrical transmission through them is dependent upon the percentage of the cross-sectional area of the supposed weld which is actually welded. Maldistribution of the amount of welded surface area from weld to weld across the face of a bipolar electrode is very difficult to avoid. Thus with maldistribution of welds, there occurs maldistribution of electric current which, like the threaded bolt problem, causes the undesired electrical "hot spots" on the membrane and "flat plate" electrodes.
Warpage is another undesired side effect of welding. Welding invariably causes warpage in the workpiece. Warpage problems may initially begin before fabrication. When working with large weldments, the individual parts themselves may not be straight, flat, smooth, etc., which will ultimately cause problems during and after fabrication. For proper alignment and positioning of parts, jigs and fixtures often are not adequate to compensate for such problems.
When working with large flat structures (such as cell bodies) the biggest concern lies with warpage that occurs due to the welding itself. Methods to correct such warpage may include heating/cooling, pressing, heating/pressing, and machining. All such methods of relieving warpage induced by welding, however, may in turn induce additional stresses in the structure and thereby cause secondary warpage in the part. These methods also increase the cost of the cell bodies.
In addition to warpage, other concerns which are common to welded structures include: (1) undesirable weld stresses within the part, (2) defective welds, (3) correcting welds which are defective, (4) examination of the weldment for flaws.
In both the all welded cell structures and the welded and bolted metal cell structures, it is difficult to maintain uniform planes between the anolyte and catholyte compartments. Consequently these non-uniform planes cause a non-uniform electrical current distribution across the active surface of the catholyte and anolyte chambers. Since the distribution of electric current is non-uniform, the electrical reactions are also non-uniform. It occurs vigorously at localized areas and thereby causes localized heating effects there, that is "hot spots".
Another problem associated with these non-uniform planes is that the anode and the cathode cannot be brought sufficiently close to each other without the fear of puncturing the membrane. Thus a large voltage loss is incurred because these electrodes can not be spaced as close to each other as desired.
All of the above leads to a shortening of the life of the electrolytic cell.
The present invention by comparison (cast unitary cell structures) has eliminated most of the problems listed above which are common to the weldment type structure and the welded and bolted structure. As a result, cell electrodes are more uniformly parallel; there is a more uniform distribution of electrical current and electrolytic reaction in the cell during operation; and the invention also provides a leakproof centerboard or central barrier.
Another undesired effect of threads and welds in titanium is that they create stress points in the titanium. These stress points are very susceptible to attack by atomic hydrogen. This attack forms significant concentrations of hydrides of titanium at temperatures greater than 80.degree. C. These hydrides are structurally unsound and resistant to the passage of electricity. Thus the purposes for which these threads or welds were made in the first place are substantially undone when hydrides are formed thereat.
The source of this atomic hydrogen is primarily the cathode compartment where water is electrolyzed to hydrogen and hydroxide. It would seem that little trouble would be expected in titanium located in the anode compartment from atomic hydrogen generated in the cathode compartment, particularly when there is a steel central barrier located between them.
However, this hydrogen diffuses through the steel and does attack titanium stress points with particular devastating results at temperatures greater than 80.degree. C., the temperature above which membrane cells coincidently seem to operate best.
The atomic hydrogen attacks the titanium stress points directly connected to the steel. This is one of the flaws in the reasoning given for using a steel to titanium explosion bonded central barrier as is disclosed and claimed in Seko, U.S. Pat. No. 4,111,779. The whole bonded area of the titanium is under stress and is therefore subject to the hydride formation discussed above. At first no problem is detected because sufficient hydrogen has not penetrated the steel and reached the titanium. However, as the titanium hydride formation increases in these central barriers at the titanium steel bond, the electrical conductivity and the structural integrity decreases.
Stabilized electrocatalysts have been used as electrodes in processes and devices for the generation of chlorine and hydrogen by electrolysis of an aqueous alkali metal halide at the anode of an electrolysis cell and of water at the cathode of an electrolysis cell which includes a solid polymer electrolyte in the form of a cation exchange membrane to separate the cell into cathode and anode compartments. The catalytic electrodes at which the chlorine and hydrogen are produced are thin, porous, gas permeable catalytic electrodes which are bonded to or embedded in opposite surfaces of the membrane so that the chlorine and hydrogen are generated (substantially) at the electrode membrane interfaces. This results in electrodes which have very low overvoltages for chlorine and hydrogen discharge.
The present invention greatly reduces the risk of titanium hydride formation by creating a structure which has a titanium liner with only a relatively very few stress points in it, and also by locating these stress points at an extreme distance from the hydrogen source with respect to the amount of steel which must be traversed in order to reach any of these few stress points. The only stress points found in the present invention's titanium hot pressed liner are found at the sites where it is welded to the ends of the integrally cast anode bosses. These will be discussed below. It should be understood here, however, that although the present invention has been discussed principally in terms of the commonly used steel and titanium, it is not limited to these materias of construction, albeit they are the preferred material of construction.